问题描述：因为objc是动态语言，对象的类型在运行时才会被确认，所以很容易出现一个定义为NSString类型的变量，在运行时的类型变成了NSNull，从而导致如下错误出现:-[NSNull stringByAppendingFormat:]: unrecognized selector sent to instance
下面介绍一下解决这个问题的思路
首先我们知道objc提供了消息转发机制，可以挽救当一个类调用了不存在的方法时，给你挽救的机会。我们就可以利用这个机制来拯救这个错误。
我们通过创建一个NSNull的分类，然后重写- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector和- (void)forwardInvocation:(NSInvocation *)anInvocation两个方法
1、在methodSignatureForSelector方法体中我们需要遍历系统中所有注册的iOS类，然后找到可以执行aSelector的类，并返回这个类的该aSelector的方法签名对象。
这里涉及以下几点：
a、遍历所有的已注册的类，利用runtime的objc_getClassList或objc_copyClassList
b、排除那些非继承自NSObject的类
c、对过滤后的类缓存，以及类对应的签名也要进行缓存，这里是为了提高查找效率
2、在forwardInvocation里如下：
1 - (void)forwardInvocation:(NSInvocation *)anInvocation{
2 anInvocation.target = nil;
3 [anInvocation invoke];
4 }
这里利用了objc的对象值为null的对象调用任何方法都会不执行并不报错的特点，将调用者target的值设置为null即可。
下面是获取方法签名的具体代码：
1 - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
2 @synchronized([self class])
3 {
4 //look up method signature
5 NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
6 if (!signature)
7 {
8 //not supported by NSNull, search other classes
9 static NSMutableSet *classList = nil;
10 static NSMutableDictionary *signatureCache = nil;
11 if (signatureCache == nil)
12 {
13 classList = [[NSMutableSet alloc] init];
14 signatureCache = [[NSMutableDictionary alloc] init];
15
16 //get class list
17 int numClasses = objc_getClassList(NULL, 0);
18 Class *classes = (Class *)malloc(sizeof(Class) * (unsigned long)numClasses);
19 numClasses = objc_getClassList(classes, numClasses);
20
21 //add to list for checking
22 c
23 for (int i = 0; i < numClasses; i++)
24 {
25 //determine if class has a superclass
26 Class someClass = classes[i];
27 Class superclass = class_getSuperclass(someClass);
28 while (superclass)
29 {
30 if (superclass == [NSObject class])
31 {
32 [classList addObject:someClass];
33 break;
34 }
35 [excluded addObject:NSStringFromClass(superclass)];
36 superclass = class_getSuperclass(superclass);
37 }
38 }
39
40 //remove all classes that have subclasses
41 for (Class someClass in excluded)
42 {
43 [classList removeObject:someClass];
44 }
45
46 //free class list
47 free(classes);
48 }
49
50 //check implementation cache first
51 NSString *selectorString = NSStringFromSelector(aSelector);
52 signature = signatureCache[selectorString];
53 if (!signature)
54 {
55 //find implementation
56 for (Class someClass in classList)
57 {
58 if ([someClass instancesRespondToSelector:aSelector])
59 {
60 signature = [someClass instanceMethodSignatureForSelector:aSelector];
61 break;
62 }
63 }
64
65 //cache for next time
66 signatureCache[selectorString] = signature ?: [NSNull null];
67 }
68 else if ([signature isKindOfClass:[NSNull class]])
69 {
70 signature = nil;
71 }
72 }
73 return signature;
74 }
75 }
原文地址：https://www.cnblogs.com/zbblog/p/12120413.html